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Weed control, management, ecology, and minutia
by Mike Pitcairn
on April 15, 2015 at 12:10 PM
Some clarification regarding this new mite. There is no official name for this species. Two Eriophyid mites that attack plants in the broom tribe have accidentally been introduced into California. One is found on gorse (Ulex europea) and French broom (Genista monsplessiana) and now this new mite on Scotch broom. The mite on gorse and French broom doesn't produce a gall but lives in the growing tips of young leaves. Both mites have been identified as Aceria genista but it is clear that these are different species based on behavior and host range. DNA analysis show them to be different too. The definitive publication providing the species descriptions and clarification is being written by a mite taxonomist in Canada but until this is published, no name exists.  
Second, no non-target host testing has been performed on this mite. As a result, there is no permit to move the mite, even within California. It is hoped that this will be done eventually.
by Jan Lockard
on August 7, 2017 at 1:21 PM
What can I do about this Genista mite? These brooms  
in my front yard are 12 years old and I don't want to lose  
them. Should I spray or prune?  
Thank you, Jan Lockard
by Scott Oneto
on August 8, 2017 at 5:28 PM
Hi Jan, First I would determine if in fact the plants in your yard are Scotch broom. This mite has only been observed on Scotch broom, so if you have another broom type, then the mite would not be a concern. If you send me some pictures of the plant in your yard, I can help identify which type of broom you have (my contact information is above). If in fact it is Scotch broom and/or you are seeing mite activity already, I would ask that you reconsider this plant choice. This plant has proven to be very invasive and is a major problem across the state. I could give you some alternative plant choices that would give you a similar look as broom that is not a weed.
by Genevieve
on April 13, 2019 at 4:06 PM
A friend gave me some scotch broom with gall mite and said to spread it among unwanted wild scotch broom on my property to slow down their growth.  
I wonder if the mites would then spread to other wild plants/trees such as oak, pine, manzanita, I would not want that to happen.  
Do you know if the gall mite attacks scotch broom only?  
by Scott Oneto
on April 17, 2019 at 8:59 AM
Hi Genevieve,  
In general these types of mites are host specific. Because this mite has never been officially released through the United States Department of Agriculture as a biological control agent, there is current work being done to verify the specificity of this mite. To date, this mite has not been destructive to any other species. With regards to spreading the mite on your property, the mites do a really good job at spreading themselves. They live inside the galls and don't emerge till the late summer when the galls begin to brown. The mites will then seek refuge in dormant buds and in the cracks of bark along the stem where they will over winter. In the spring, as the weather warms and the plants begin to grow, the mites emerge and are spread by wind currents to other areas.
by Charles Wilson
on July 26, 2021 at 5:38 PM
"Because this mite has never been officially released through the United States Department of Agriculture as a biological control agent, there is current work being done to verify the specificity of this mite."  
It's been >2 years since you wrote that, and 7 or 8 years since this mite was observed. Has there been more study? Can you describe the USDA's release process in general, and specifically what has (or has not) happened with this particular situation?  
Scotch Broom is a sigificant issue in CA, OR, and WA. How is the USDA responding?
by Scott Oneto
on July 27, 2021 at 3:37 PM
Hi Charles, Great questions. The USDA ARS (Agriculture Research Service) out of Albany CA, has been working hard since 2015 to study the Scotch broom gall mite. The lead scientist for this project is Dr. Paul Pratt. He has undergone several studies and I have referenced them below. Paul also just gave a great presentation on this topic back in January and a link to that presentation is below. In this presentation Dr. Pratt highlights all the studies they have conducted over the past several years and their findings.  
Dr. Pratt has conducted several environmental studies to determine if the mites would develop galls on other plants. To date, there have been no known occurrences of mites forming galls on native plants and several other tested perennial plants and many species of lupines. One of his final conclusions is that evidence to date suggests that the Scotch broom gall mite is very host specific and is safe to redistribute. However he also goes on to say that a federal permit is still currently required to spread the mite. So my take home message is let the mite be, as it seems to be spreading pretty good on its own. The mite spreads in wind currents, and I most recently found the mite in Reno, NV which I believe is the first recorded finding in that state.  
If you are interested in reading more about the projects Dr. Pratt has conducted, he just published some work looking at a prediction distribution study. That study can be found online:  
Invasion of the gall mite Aceria genistae (Acari: Eriophyidae), a natural enemy of the invasive weed Cytisus scoparius, into California, U.S.A. and predictions for climate suitability in other regions using ecological niche modelling
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